As a lithium-ion secondary battery has a large electrochemical capacity, a high operating potential and excellent charge-discharge cycle characteristics, the demand of the lithium-ion secondary battery has been increased to be used for mobile information terminals, mobile electronic devices, small household power storage devices, motorcycles using a motor as a power source, electric vehicles, hybrid electric vehicles, and the like. According to the increase in the usage, the lithium-ion secondary battery has been required to have improved safety and high performance. However, since the existing lithium-ion secondary battery using a nonaqueous electrolyte in which lithium salt is dissolved in an organic solvent, as an electrolyte, is easily ignited at a temperature of about 150° C., safety issue may arise.
Accordingly, in recent years, an all-solid secondary battery using a solid electrolyte including a noncombustible inorganic material has been actively researched for the purpose of improving safety.
The all-solid secondary battery using the solid electrolyte includes an electrolyte layer including solid electrolyte, and a cathode layer and an anode layer each including solid electrolyte. The cathode layer and the anode layer are formed on both surfaces of the electrolyte layer, and current collectors are combined with each electrode.
A compression molding method has been used as a manufacturing method of the all-solid secondary battery. For example, the compression molding method includes sequentially adding and pressing component powders of a battery in a cylindrical mold, and combining current collectors on a cathode and an anode. However, the compression molding method has a problem in that since it is difficult to uniformly deposit the cathode layer on the electrolyte layer, uniform current distribution in the cathode is not formed, such that deviation of current density may be caused, which may reduce performance of a secondary battery. In addition, since a mold or a device required according to area of the electrode to be manufactured is separately required, problems in view of process may occur.
In order to overcome the above-described problems of the manufacturing method of the all-solid secondary battery, in the related arts, a method of casting a slurry in which an electrode active material is mixed with a binder and a solvent on a current collector, followed by drying, and sequential lamination casting thereon, and a method of mixing electrode active materials of each layer with a binder and a solvent, followed by casting to form a thick film, and lamination process adhering each sheet, have been reported. According to these manufacturing methods, uniform electrodes may be formed and coating area may be increased, thereby increasing area of the electrode.
However, since sulfide-based solid electrolyte has high reactivity to moisture and polar materials, when applying a binder solution used in the existing lithium secondary battery, the method of preparing electrode active material slurry and casting the slurry on the current collector may have a problem. Therefore, at the time of preparing the slurry for manufacturing the thick film, a binder and a solvent that do not react with the sulfide-based solid electrolyte are required to be used.
In addition, at the time of adding the binder and the solvent for forming the thick film, when a content of the binder required for forming adhesion between the current collector and the electrodes is small, the current collector and the electrodes may be delaminated, and when the content of the binder is large, resistance in the electrodes may be increased, which may reduce electrode properties.
Therefore, an electrode slurry capable of strongly adhering the current collector while reducing the content of the binder, thereby excellently maintaining electrode properties, and a manufacturing method of an all-solid secondary battery have been demanded.